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United States Patent |
6,077,439
|
El-Ammouri
,   et al.
|
June 20, 2000
|
Method for the removal of metals from solution by means of activated
silica
Abstract
The surprising ability of the polysilicate microgels commonly known as
"activated silica" to adsorb and to release heavy metals selectively as a
function of pH is used in a novel method for separating metals from dilute
aqueous solution by means of selective precipitation with and recovery
from an activated silica absorbent, selected changes to the pH. The
process is particularly intended for the economic purification of
contaminated waste streams to recover valuable but toxic heavy metals from
such effluents at a lower cost than hitherto possible, using an activated
silica substrate which may be regenerated by alkali treatment.
Inventors:
|
El-Ammouri; Elias Gebran (Montreal, CA);
Distin; Philip Andrew (Hudson, CA);
Lempka; Barbara Mary-Ann (Mississauga, CA);
Hagens; Rodger Graham (Hamilton, CA)
|
Assignee:
|
McGill University (Montreal, CA)
|
Appl. No.:
|
916417 |
Filed:
|
August 22, 1997 |
Current U.S. Class: |
210/665; 210/670; 210/688; 210/912 |
Intern'l Class: |
C02F 001/28; C02F 001/64 |
Field of Search: |
210/665,670,684,688,711,716,912,681
|
References Cited
U.S. Patent Documents
3963640 | Jun., 1976 | Smith | 210/716.
|
4396585 | Aug., 1983 | Rosene | 210/684.
|
Primary Examiner: Cintins; Ivars
Attorney, Agent or Firm: Ridout & Maybee
Parent Case Text
RELATED APPLICATION
This application replaces U.S. Provisional Patent Application No.
60/024,498, from which it derives the benefit of a filing date of Aug. 23,
1996.
Claims
We claim:
1. A method for the selective separation of metal ions from a dilute
aqueous solution of a mixture of at least two kinds of metal characterized
by differing pH-dependent adsorption affinities for complexing with
activated silica, comprising the steps of:
(a) adding to said dilute aqueous solution sufficient activated silica and
raising the pH of the solution to a first value, at which substantially
all of a first kind of metal ion present complexes selectively with the
activated silica to form a first precipitate therewith, the remaining
kinds of metal from said dilute aqueous solution remaining in a first
supernatant liquid layer;
(b) physically separating said first precipitate from said first
supernatant liquid layer;
(c) treating said first precipitate with mineral acid to lower the pH
sufficiently to effect desorption of said first kind of metal into a
supernatant layer of concentrated solution, then physically separating a
concentrated solution of said first kind of metal from the activated
silica;
(d) treating the separated first supernatant liquid layer from step (b)
with sufficient alkali to raise the pH to a second value and with
sufficient activated silica, such that substantially all of a second kind
of metal ion preferentially complexes with the activated silica and forms
a second precipitate therewith, any other kinds of metal ion originally
present in said dilute aqueous solution of at least two kinds of metal ion
remaining in a second supernatant liquid layer; and
(e) physically separating said second precipitate from said second
supernatant liquid layer and treating the second precipitate with mineral
acid to lower the pH sufficiently to effect desorption of said second kind
of metal into a concentrated solution thereof.
2. A method according to claim 1, wherein the activated silica used in step
(d) to complex with said second kind of metal is activated silica
separated in step (c) from said concentrated solution of the first kind of
metal and then regenerated.
3. A method according to claim 1, wherein the dilute aqueous solution of
metal ions comprises ferric and aluminum ions as said first kind of metal
ion, heavy metals as the second kind of metal ion and alkali and alkaline
earth metals as a third kind of metal and wherein:
(a) the pH is lowered in step (c) to below about 2, thereby to release the
adsorbed first kind of metal into said mineral acid to produce a
concentrated solution of ferric and aluminum ions; and
(b) in step (d), sufficient alkali is added to produce a second pH value of
about 7 to 8, effecting the formation of a second precipitate containing
said heavy metals and a second supernatant liquid layer containing said
alkali and alkaline earth metals.
4. A method according to claim 3, comprising the further step of treating
said second precipitate after its separation from said second supernatant
liquid layer with mineral acid to reduce the pH to below about 5 and
effect the desorption and release of said heavy metals as soluble salts in
solution.
Description
FIELD OF THE INVENTION
This invention relates generally to the use of activated silica to remove
metal ions from aqueous solution by precipitation, to the subsequent
recovery of the metal ions in concentrated form from the precipitate by
acidification, and to the separation of dissolved metals by means of
precipitation with activated silica solution followed by selective
recovery of metal ions from the precipitate at different pH conditions. In
particular, the invention relates to a method for recovering heavy metals
from industrial waste streams, such as the aqueous run-off from mining
operations.
BACKGROUND OF THE INVENTION
The contamination of the environment with aqueous solutions of heavy metals
from industrial waste stream remains a serious problem. In spite of
significant advances in the treatment of such effluent in recent years, no
completely satisfactory method for the removal of such toxins from
industrial effluent yet exists. One area of particular concern is the
aqueous run-off from mining operations, including so called "acid mine
drainage." The effluent from mining operations frequently contains a
complex mixture of heavy metals such as copper, nickel, lead, zinc, etc.
at concentrations well above the acceptable regulatory limits, but too low
to be economically recovered by conventional processes. These streams
which are usually at a low pH, also contain large quantities of other,
less toxic metals such as iron and calcium.
Currently, the method most widely used to remove toxic metals from effluent
streams involves raising the pH of the solutions to the level at which the
metal hydroxides are least soluble, usually between 9 and 11, so that they
can be removed by precipitation. The best technology hitherto in use
employs hydrated lime as an alkali source, with precipitation of the metal
hydroxides in large clarifiers facilitated by additional sedimentation
aids such as ferric sulfate and organic polymers. This treatment results
in the recovery of large quantities of a voluminous sludge consisting of a
mixture of the metal hydroxides admixed with calcium sulfate (gypsum).
Since the metals are unrecoverable from this gypsum matrix, the sludge is
then transferred to a landfill site, or returned to the tailings pile from
which the metals had originally emanated. This procedure is undesirable
for a number of reasons. For one, the steadily rising costs of landfill
make the disposal of toxic sludge ever more expensive. The second problem
is that the metals are not permanently removed from the environment. This
is because the chemical environment within the landfill itself is usually
unstable, and subject to a steady decline in pH. As this occurs the metal
hydroxides re-dissolve and re-enter the environment, requiring yet another
treatment. Since mine tailings and landfill sites are likely to remain in
place for many centuries, such an ongoing cycle is clearly unacceptable.
Another factor contributing to the desirability of extracting the heavy
metals from the effluent is the fact that in their pure form the metals
are of considerable economic value. Not unexpectedly, much attention has
been given in recent years to methods which might allow the recovery of
the dissolved metals from solution. Some techniques which have been taught
to achieve this end include the use of membrane filtration, or
electrochemical methods [L. L. Tavlarides et al. Separation Sci &
Technology 22: 2-3 (1987)] These methods, however, suffer the disadvantage
of high operational cost, and are inadequate for the large volumes of
liquid commonly encountered in mine effluent streams. Another method which
has enjoyed some success in the removal (but not recovery) of the toxic
metals involves insolubilization of the gypsum sludge by the formation of
a cementitious matrix, the so-called "Chemfix" process. (R. B. Pojesek
Chem Eng. 86, Aug. 13, 1979; P. G. Lawrence Chem-fix Inc. Report 1980,
Pittsburgh, Pa.). This method has not however received widespread
application since the treatment is not only expensive, but the long term
validity remains unproven. The use of soluble alkali silicates for the
removal of heavy metals from solution is described by J. S Falcone (ACS
Symposium Ser. 194 Am. Chem Soc. New York, 1982), but this too results in
the formation of a complex precipitate from which the metals cannot be
economically extracted.
Various chemical methods to recover metals from waste streams by ion
exchange have been described. Thus zeolite (M. J. Zamzow et al. Sep. Sci.
Technology 25: 13-15 (1990) 1555-69), quartz (T. W. Healy et al. Adv.
Chem. Ser 79 (1968) 62) and alumina (M. Uberoi and F. Shadman Prep. Pap.
Am. Chem Soc. Div Fuel Chem 4 (1991) 36) have all been recommended for
this purpose. Although each of these methods offers the promise of
recovering the metals from solution, they all require relatively high
concentrations of metals in solution to be effective. The German Patent
disclosure DE 42 44 258 A1 (Jun. 23, 1994; Grace GmbH) on the other hand
teaches that silica gel can be used to concentrate cadmium in solution
from quite dilute solutions, but this method is relatively slow, and
suffers from a number of other disadvantages for which silica gel is well
known.
The ability of silica gel to remove metals from solution by selective
adsorption has been extensively described, and the adsorption and
desorption of a wide range of metals under different pH conditions has
been studied by D. L. Dugger et al. (J. Phys. Chem. 68 (1964) 757060), R.
O. James and T. W. Healy (J. Coll. Interface Sci. 40 (1972) 65-81), V. F.
and J. Galba Coll (Czech Commun. 32 (1967) 3, 530-6), and P. W. Schindler
et al. (J. Coll Interface Sci. 55 (1976) 469-75). A theoretical discussion
of the adsorption of dissolved metals by silica is also to be found in R.
K. Iler ("The Chemistry of Silica," New York: John Wiley & Sons, 1979).
One of the problems of using silica gel as an ion exchange medium,
however, is that being a weak acid, the pH of the solution from which the
metals are removed declines as the metals are adsorbed onto the silica
gel. This has the consequence that as the process proceeds desorption
begins to occur. Even if the pH were to be artificially controlled in
order to effect selective adsorption of metals from solution (as would be
obvious to those skilled in the art), another problem arises due to the
fact that silica gel is physically quite fragile and expensive. The
handling of the product which is required to facilitate the process can
frequently lead to destruction of the gel, at considerable financial cost.
Various recent disclosures have sought to avoid some of these drawbacks by
adsorbing various organic and inorganic ion exchange materials onto either
alumina or silica gel, which such cases are treated as an inert substrate.
Thus the German patent disclosure DE 3823957 A1 (Jan. 18, 1990, R.
Ballhorn) teaches that heavy metals can be removed from solution by means
of calcium phosphate adsorbed onto silica gel. Similarly Schlapfer (C. W.
Schlapfer, U.S. Pat. No. 5,102,640 Apr. 7, 1992) describes a process for
the removal of metal ions from solution by means of dipicolylamine bound
to a silica gel surface while G. Giraudi et al. (Annali di Chemica 74
(1984) 307-13) present a method of concentrating metal ions using
pyridylazo naphthol adsorbed on silica gel. A summary of some other
complex forming reagents supported on silica gel which have been tried for
this purpose was published K. Terada (analytical Sciences 7 (1991)
187-98). Such methods, although valuable for analytical purposes are,
however, inadequate for use with industrial waste and process streams. The
coated silica gel materials described in these articles are both too
expensive and fragile for the numerous cycles required to allow this
method to compete with alternate treatments such as lime.
SUMMARY OF THE INVENTION
In our investigation of the properties of the modified alkali silicates
commonly described as "activated silica" (or, alternatively, "polysilicate
microgels"), we discovered that these materials exhibit a surprising
ability to adsorb and subsequently release heavy metals in a way which
affords advantages for the chemical recovery of metals from waste streams
over the prior art chemical systems described above.
Activated silica is a well-known material which may be described as a
highly dispersed polymeric form of silica produced when dilute aqueous
solutions of alkali metal silicates are reacted with mineral acids, or
with multivalant metal ions such as calcium, iron or aluminum.
Descriptions of the chemistry and method of preparation of activated
silica are well covered in the literature. The chemistry is to be found in
Iler (1979, p. 231), J. G. Vail ("Soluble Silicates," Vol II New York:
Reinhold, 1960), K. R. Lange and R. W. Spenser (Envir. Sci. and Technology
2: 3 (1968) 212-6), and T. Hasekawa et al. (Water Science and Technology
23 (1991) 1713)-1722). The manufacture of this material is described by C.
Henry (J. Am. Water Works Ass. 30:1 (1958) 61-71), and is disclosed in
U.S. Pat. No. 3,963,640 (Jun. 15, 1976 to Anglian Water Authority, U.K.
and No. 4,147,657 (to the PQ Corporation).
Activated silica may be manufactured and stored using specialized
equipment, or can be prepared in situ, by reacting soluble silicate with
acids or polyvalent cations in the presence of the medium to be treated by
the activated silica. The material has been used for almost sixty years in
the commercial purification of drinking water and has found application in
the flocculation of alumina and silver bromide sols and also as a
retention/drainage aid in papermaking.
However, we found no description of any special ability of activated silica
to remove metals from solution by comparison with silica gel, alumina or
other substrates, so were surprised in the course of our experimentation
to find activated silica to be not only far more effective than lime,
alkali silicate or silica gel in the removal of metal ions from solution,
but also to afford a route to regenerating the metals in concentrated form
with high efficiency. We have also found that the process of metal removal
by activated silica can be extended to allow re-use of the activated
silica itself and to selectively discriminate between different metal
ions.
According to a first aspect of the present invention there is provided a
method for removing metal ions from solution by treatment with an aqueous
solution of activated silica.
According to a further aspect of the invention, there is provided a method
for recovering in concentrated form the metal ions precipitated with
activated silica, by acidulation of the precipitate.
According to a further aspect of the invention, there is provided a method
for re-use of activated silica which has been used to precipitate metal
ions as aforesaid.
According to a further aspect of the invention, there is provided a method
of separating metals by means of selective precipitation and recovery as
aforesaid, carried out at different pH conditions.
It is a particular object of the present invention to provide a process for
using activated silica to purify contaminated waste streams and recover
heavy metals from such effluents at a lower cost and with higher
efficiency than by using silica gel as an ion exchange medium.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
(i) Efficient Removal of Dissolved Metals from Aqueous Solution by Means of
Activated Silica
The ability of activated silica prepared in situ, by the reaction of sodium
silicate and calcium hydroxide, to remove dissolved metals from solution
much more efficiently than can be achieved by the use of either of these
individual reagents alone is illustrated in Example 1 and Table I below.
The reaction between the activated silica and the metal ions results in the
formation of a precipitate which rapidly settles to the bottom of the
reaction vessel, leaving a supernatant having a very low metal content.
This might not itself afford commercial metals recovery value, as the
metals contained in the precipitated sludge remain admixed with calcium in
a manner similar to the result when calcium hydroxide alone is used as the
precipitant. However, enhanced commercial viability of the process
according to the present invention is illustrated in Examples 2, 3 and 4
below, showing the precipitated metal (copper or iron) may be readily
dissolved by the simple expedient of reducing the pH of the complex from
its initial value of about 8-9 to about 5.
It was seen that acidification of the precipitate leads to the immediate
formation of two distinct layers, one consisting of a concentrated
solution of metal ions, and the second a finely dispersed suspension of
silica. Now in concentrated form, the metals are easily separated from the
particulate silica by conventional mechanical means such as filtration or
centrifugation.
EXAMPLE 1
Efficient Removal of Dissolved Metals from Aqueous Solution by Means of
Activated Silica
A stock solution containing nickel, lead, copper and zinc was prepared by
addition of analytical grade nitrate salts of these metals to deionized
water. The amounts added were calculated to yield the concentrations shown
in Table I. Separate 100 ml samples taken from the stock solution were
mixed with either: (1) sodium silicate (in which the weight ratio of
silica to soda was 2.0), or (2) slaked lime (Ca(OH).sub.2), or (3) an
equimolar mixture of (1) and (2). The quantity of reagent added was that
required to achieve the pH range shown in Table I. The reacted samples
were left to stand for 10 days, with no further treatment except for
periodic testing of the pH. The samples were then filtered (42 analytical
grade filter paper), and the filtrate analyzed for metals by Inductively
Coupled Plasma Emission Spectroscopy. All metal concentrations shown in
the Table are expressed in parts per million. Those given at pH 5.5 are to
be taken as control.
The results illustrated in Table I reveal that the application of sodium
silicate is superior to lime in the reduction of the concentration of
heavy metals, but that the combination of the two, i.e., the in situ
formation of activated silica is much superior to both. Note in particular
that low metal concentrations are observed even when the alkalinity of the
solutions exceeds pH 9-10, a condition when the metal concentration
typically rises due to increased solubility of metal hydroxides.
TABLE I
__________________________________________________________________________
Note: "Si" designates, addition of sodium silicate alone, (1) above; "Ca"
is addition of line
alone, (2) above; and "Si + Ca" is the combination of the two (3).
.rarw. Nickel .fwdarw.
.rarw. Lead .fwdarw.
.rarw. Copper .fwdarw.
.rarw. Zinc .fwdarw.
pH Si Ca Si + Ca
Si Ca Si + Ca
Si Ca Si + Ca
Si Ca Si + Ca
__________________________________________________________________________
5.5
56.90
56.90 46.80
46.80 54.50
54.50 52.50
52.50
6 53.00
56.60 23.70
42.30 2.45
47.10 44.60
44.60
7 35.80
52.80
32.10
0.13
0.93
0.83
0.13
0.82
0.10
7.90
45.00
3.34
7.5
11.20
39.70 0.01
0.51 0.03
0.48 1.35
22.60
8 10.20
15.30 0.12
0.07 0.15
0.05 3.29
1.42
8.5
6.93
5.85
0.12
2.89
0.10
0.02
2.46
0.07
0.05
3.91
1.48
0.05
9 6.44
0.062 0.53
0.01 0.07
0.02 2.04
0.04
10 56.70 0.04
48.60 0.02
53.50 0.02
52.00 0.04
11 39.70 27.50 29.20 33.80
__________________________________________________________________________
EXAMPLE 2
Recovery of Metal Ions (Copper) in Concentrated Form
A stock solution of activated silica was prepared from a sodium silicate
solution containing 8.90 weight % soda and 28.7 weight % silica. After 50
volume % dilution with water, gelation was initiated by adding, with
stirring, 10 weight % sulphuric acid solution to give 2.04 weight %silica
solution at pH 8.20. Gelation was arrested by dilution with 50 volume %
water after half the total gel time of 18 minutes had elapsed. This
stabilized sol was the source of activated silica used in Examples 2-7.
An aqueous solution containing 63.5 ppm copper and 11 ppm iron (ferric) at
pH 5.50 was prepared from analytical grade copper and ferric sulphates. A
15 mL sample of activated silica, prepared as described above, was added
incrementally to 100 mL of copper/iron solution, with stirring, followed
by addition of a few drops of sodium hydroxide solution to reach a final
pH of 7.05. The system was held for 15 minutes to allow formation of
metal-hydroxy complexes which adsorbed onto the activated silica to form a
green opaque layer that settled beneath clear residual solution.
After centrifuging to promote rejection of water from the metal-containing
activated silica layer, the residual solution, containing 1.6 ppm copper,
was discarded. The pH of the activated silica layer was adjusted to 4.36
using 1.0 weight % sulphuric acid solution. After a contact time of 20
minutes, the system was centrifuged again, giving an essentially iron-free
aqueous phase containing 355 ppm copper. The activated silica layer is now
brown, indicating that iron remains adsorbed.
It should be noted that, although it is possible to concentrate copper in
the above manner, but from an iron-free feed solution, the stability of
the activated silica layer is enhanced by the presence of iron-hydroxy
complexes during and after copper desorption. If copper is re-dissolved as
described above, but leaving activated silica totally devoid of hydroxy
complexes, the activated silica will tend to disperse, leading to silica
losses into the product solution and/or effluent if recycled.
EXAMPLE 3
Recovery of Metal Ions (Iron) in Concentrated Form
An aqueous solution containing 558 ppm iron (ferric) at pH 2.45 was
prepared from analytical grade ferric sulphate. A 20 mL sample of
activated silica, prepared as described in Example 2, was added
incrementally to 100 mL of iron solution, with stirring, followed by
addition of 2.9 mL of 28.0 g/L slaked lime solution to reach a final pH of
4.00 after 24 hours retention.
After centrifuging, the residual solution (effluent) containing 1.9 ppm
iron, was discarded. The pH of the activated silica layer was adjusted to
0.77 using a few drops of concentrated (93.1 weight %) sulphuric acid.
After a contact time of 20 minutes, the system was centrifuged again,
giving a clear brown aqueous phase occupying about 75% of the total volume
and containing 8.00 g/L iron (product solution). An opaque, white lower
layer of activated silica was retained, which occupied about 25% of the
total volume and was essentially iron-free. The iron distribution after
the adsorption/desorption cycle is 99.6% into the product solution, 0.4%
into the effluent and 0.0% retained by the activated silica. Although
essentially complete iron recovery at a concentration factor of 14.3 (i.e.
558 ppm Fe feed, 8.00 g/L product) is achieved, an iron-free activated
silica layer at pH 0.77 has been created which tends to disperse so
increasing silica losses into the product solution and/or effluent. Silica
losses into the effluent after adsorption and into the iron solution after
desorption were 4.1% and 9.0% of the initial activated silica addition
respectively.
EXAMPLE 4
Recovery of Metal Ions (Iron) in Concentrated Form
An aqueous solution containing 11.1 g/L iron (ferric) at pH 1.45 was
prepared from analytical grade ferric sulphate. A 40 mL sample of
activated silica, prepared as described in Example 2, was added
incrementally to 100 mL of iron solution, with stirring, followed by
addition of 56.0 mL of 28.0 g/L slaked lime solution to reach a final pH
of 2.82 after 24 hours retention.
After centrifuging, the residual solution (effluent) containing 97 ppm
iron, was discarded. The pH of the activated silica layer was adjusted to
1.35 using 1.0 mL concentrated (93.1 weight %) sulphuric acid. After a
contact time of 1 hour, the system was centrifuged again giving a clear,
dark brown aqueous phase occupying about 75% of the total volume
containing 39.3 g/L iron (product solution). An opaque, light brown, lower
layer of activated silica was present, which occupied about 25% of the
total volume, and contained 120 ppm iron. The iron distribution after the
adsorption/desorption cycle was 84.6% into the product solution, 14.0%
retained by the activated silica, 1.4% into the effluent.
(iii) Regeneration of Activated Silica following Recovery of Precipitated
Metal Ions
The recovery of metal ions according to the process of the present
invention is rendered still more economical by the capability which
activated silica affords for regeneration and re-use in subsequent metal
precipitation/separations. This regeneration can be accomplished by the
simple expedient of raising the pH of the residual silica obtained after
the dissolved metals have been physically removed. This can be achieved by
the addition of one or more sources of alkali, as illustrated in Example
5.
EXAMPLE 5
Re-use and Recycling of Activated Silica (Copper Recovery)
An aqueous solution containing 63.5 ppm copper and 11 ppm iron (ferric) at
pH 3.43 was prepared from analytical grade copper and ferric sulphates for
use in two consecutive adsorption/desorption cycles. In the first cycle,
22.5 mL of activated silica, prepared as described in Example 2, was added
incrementally to 100 mL of copper/iron solution, with stirring, to reach a
final pH of 7.07 after 15 minutes retention.
After centrifuging, the residual solution contained 2.3 ppm copper, which
represents a loss of 3.5% of the feed copper. After discarding the
residual solution, the pH of the activated silica layer was adjusted to
4.13 with 1.0 weight % sulphuric acid solution. After a contact time of 20
minutes, the system was centrifuged again, giving a clear aqueous phase
containing 245 ppm copper, 28 ppm iron. An opaque, light-brown lower layer
of activated silica containing adsorbed iron was present, which was then
used in a second cycle.
In the second cycle, the recycled activated silica layer was added
incrementally to 100 mL of copper/iron solution, with stirring, and a few
drops of sodium hydroxide solution were required to reach a final pH of
7.05 after 15 minutes retention. After centrifuging, the residual solution
contained 1.9 ppm copper, which represents a loss of 3.2% of the feed
copper. After discarding the residual solution, the pH of the activated
silica layer was adjusted to 4.15 with 1.0 weight % sulphuric acid
solution. After a contact time of 20 minutes, the system was centrifuged
again, giving a clear aqueous phase containing 420 ppm copper, 17 ppm
iron. A brown lower layer of activated silica was generated, as seen at
the end of the first cycle.
It is understood that if only copper is being recovered from a copper/iron
solution using multiple recycles of activated silica, there will be a
progressive build-up of adsorbed iron, which would eventually have to be
removed by desorption at pH 1-2.
(iv) Separation of Metals by Means of Selective Precipitation and Recovery
The full potential for the use of activated silica in the purification of
acidic waste streams becomes evident in the aspect of this discovery
illustrated in Examples 6 and 7 below, relating to the use of activated
silica in separating metals having different adsorption profiles with
respect to activated silica. This is effected by selective pH control, in
a manner analogous to that in which conventional ion exchange materials
have been employed.
Because the microstructure of activated silica is not dissimilar to that of
silica gel itself, one would anticipate the pH dependence of the reaction
between dissolved metals and activated silica to be similar, although not
identical to the case of silica gel. In keeping with the references cited
above (Dugger 1964; James & Healy 1972; Schindler 1976) it might be
expected that ferric and aluminum cations would react with activated
silica at a relatively low pH, while a number of common heavy metals would
be expected to adsorb onto activated silica at a somewhat higher pH,
perhaps between pH 6 and 8. Calcium and magnesium might be expected to be
removed from solution only at pH in excess of 8.5.
As shown in the following examples, activated silica was found in fact to
be capable of separating two metals of widely differing adsorption
profiles, the method being demonstrated with ferric and cupric ions in
Example 6 and with nickel and magnesium ions in Example 7. Evidently,
effective separation of iron, aluminum, calcium and magnesium from the
heavy metals by selective adsorption, would present a very valuable
feature in the recovery of these metals.
Conceptually, the removal and recovery of metals from a waste source
containing a complex mixture of metals and acid according to an embodiment
of the present invention can be described in the following steps, each
readily achievable by engineers skilled in the art of chemical processing:
(1) Reaction 1: To the waste material being treated, which might typically
be acidic with a pH between about 2 and 5, and contained in a reactor, add
sufficient activated silica (pH 8-9) to complex all the ferric and
aluminum ions present. Experience has shown that maximum adsorption is
achieved when the ratio of silica (S.sub.i O.sub.2) to metal ion is about
10:1. In this step care should be taken that the final pH of the reactants
not exceed about 6. This treatment will result in the rapid settling of a
precipitate containing silica and adsorbed ferric and aluminum ions, with
the other heavy metals being retained in the supernatant.
(2) The ferric/aluminum/silica precipitate is then separated from the
supernatant by one of the well known methods used for this process (e.g.
filtration or centrifugation). The supernatant (now containing the heavy
metals) is removed for the next treatment stage.
(3) Reaction 2: The precipitate obtained from Reaction 1 is then treated
with mineral acid to below pH 2 in order to release the adsorbed metals
[the method described in Examples 2 and 3]. The concentrated solution
containing ferric and aluminum ions is recovered for sale, or further
processing, while the silica recovered is transferred to another reactor.
(4) The regenerated activated silica is then available for re-use.
Optionally, an alkali source, preferably, but not limited to sodium
hydroxide, is then added to the silica in the second reactor so that the
pH is increased to between 10 and 12.
(5) The supernatant obtained from Reaction 1, is then reacted with an
alkali source, preferably but not limited to lime, to raise the pH to 7-8,
after which sufficient activated silica is added to adsorb the heavy
metals, care being taken that the final pH of the reaction not exceed
about 8.5.
(6) As before, the two layers which form are isolated from each other, and
the supernatant layer (now containing only sodium, calcium, magnesium and
sulfate ions at pH 8-9, can be discharged as effluent.
(7) The lower layer is then treated with mineral acid to reduce the pH to
below about 5, at which stage the mixture of heavy metals (e.g. Cu, Ni,
Fb, Zn) are released in the form of their soluble salts. One of a number
of mineral acids can be used to effect this low pH. Typically the acid of
choice will be one of (though not limited to) sulfuric, hydrochloric or
nitric acids, the one chosen will depend on such factors as the
composition of the heavy metals, availability, etc.
(8) The next step involves separation of the dissolved metals from the
silica, which now appears in the form of dispersed particulate silica, by
centrifugation or settling. The supernatant solution containing the heavy
metals now in concentrated form can be sent either to recovery, via a
process such as electrorefining, or if the concentration is still
considered too low, it can be returned to the beginning of the process for
further upgrading.
(9) Optionally, the dispersed silica is treated with an alkali metal
hydroxide or carbonate in order to raise the pH to 10-11, at which time it
is regenerated in the form of activated silica and available for re-use.
Turning now to specific experimental examples of metals removal and
recovery according to the present invention:
EXAMPLE 6
Metals Separation (Copper/Iron) by Selective Precipitation
An aqueous solution containing 63.5 ppm copper and 55.8 ppm iron (ferric)
at pH 3.09 was prepared from analytical grade sulphate salts. After
addition, with stirring, of 5 mL of 0.84 g/L slaked lime solution to 100
mL of copper/iron solution, activated silica, prepared as described in
Example 2, was added in amounts as needed to reach the pH values shown in
Table II. The activated silica formed a distinctively coloured lowered
layer containing hydroxy complexes of iron (brown), copper (blue) or
copper and iron (green), while the upper layer of residual solution was
essentially colourless.
After 12 hours retention, the system was centrifuged, and residual solution
was analyzed for copper and iron contents, as shown in Table II. Optimum
separation is at about pH 5, at which at least 90% of the iron has been
adsorbed onto the activated silica, while most of the copper remains
dissolved.
TABLE II
______________________________________
Copper/Iron (Ferric) Separation Using Activated Silica
pH ppm Fe ppm Cu
______________________________________
3.09 55.8 63.5
4.23 17.5 57.0
5.20 2.4 48.5
6.20 0 20.0
7.03 0 2.15
______________________________________
EXAMPLE 7
Metals Separation (Nickel/Magnesium) by Selective Precipitation
An aqueous solution containing 3.0 g/L nickel and 10.0 g/L magnesium at pH
3.30 was prepared from analytical grade sulphate salts. After addition,
with stirring, of 20 ml of activated silica, prepared as described in
Example 2, 28.0 g/L slaked lime solution was added in amounts as needed to
reach the pH values shown in Table 2. The activated silica formed a
distinctively coloured lower layer containing hydroxy complexes of nickel
(green) or nickel (green) and magnesium (white), while the upper layer of
residual solution was colourless when nickel-free. After 1 day retention,
the system was centrifuged, and residual solution was analyzed for nickel
and magnesium contents, as shown in Table III. Optimum separation is at
about pH 8.2, at which about 95% of the nickel has been adsorbed onto the
activated silica, while 83% of the magnesium is retained by the residual
solution.
TABLE III
______________________________________
Nickel/Magnesium Separation Using Activated Silica
pH ppm Ni g/L Mg
______________________________________
3.30 3,000 10.0
7.17 2,775 9.14
7.59 1,942 8.71
7.98 494 8.57
8.22 155 8.29
8.50 4.9 7.50
9.55 0.9 6.39
10.1 0.3 2.29
______________________________________
The use of activated silica to purify contaminated waste streams and
recover heavy metals from such effluents therefore has a number of novel
advantages:
(1) Low cost: Activated silica is readily prepared by treating low cost
commercial alkali (most commonly sodium) silicate, with common mineral
acids.
(2) Ease of handling: Because both the activated silica and its complex
with metal ions remain in the form of a pumpable slurry, the loss of
silicas by particle destruction, a serious drawback in the case silica gel
is eliminated.
(3) Efficiency: The loss of material due to the friable nature of silica
gel is avoided.
(4) Recyclability: The economics are improved by the fact that nearly all
the silica can be recycled, and only small amounts of fresh alkali
silicate are required to compensate for process losses.
(5) pH control: The drop in efficiency due to pH decline which occurs when
silica gel is used as an ion exchange medium is eliminated.
The invention therefore offers of a superior method of the treatment of
waste water streams containing toxic metals which is both efficient and
cost effective. In this method and reaction of activated silica with
dissolved metals can be effectively clean the effluent and concentrate the
heavy metals in a form in which they are readily recoverable.
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